This application is related to an application entitled xe2x80x9cCombination Clock and Charge Pump for Line Powered DAAxe2x80x9d and an application entitled xe2x80x9cEvent Detection Circuitxe2x80x9d, both of which are assigned to the assignee of the present invention and both of which were filed concurrently with this application.
This invention generally relates to the field of telecommunication networks, and more particularly to a telephone line interface circuit with event detection capabilities.
Telephone networks comprise a series of interconnected subsystems that are linked together at points called interfaces. These interfaces provide links between different equipment in the network and allow for simplified design and maintenance. A local loop is an example of an interface that connects a subscriber""s telephone set (device side) and the central office (line side).
Many portable computer devices also utilize modems and other data devices for communicating over a telephone line. The battery source, which powers both the portable computer device and its modem, is typically sized for general computing applications, and runs out of power quickly when actively communicating over a telephone line through a modem. Thus, although portable computer devices operate for a sufficient length of time for quick data transfers over a modem powered completely by a battery, they typically require that external AC power be supplied to allow for longer uses of the modem. It is therefore desirable for battery powered computer devices including a modem to draw power, in addition to the inherent battery, from a secondary power source.
The DC power inherent in a telephone line provides a convenient source of power, but there are often limitations and restrictions which limit the ability of a modem to derive power from the telephone line. For instance, present regulations in the United States require that significant current may only be drawn from the telephone line when the telephone or modem is in an off-hook or active condition. In order to hold the telephone line in an off-hook condition, current in the approximate range of 20 milliamps (MA) to 150 MA must be drawn.
The U.S. Federal Communication Commission (FCC) and other counterpart regulatory agencies in other countries also require electrical isolation between the line side and the user devices on the user side. The user devices include telephone sets and computers using modems. Electrical isolation protects the line side from damage transmitted from the device side and vice-versa.
Many components (e.g., data access arrangements (DAAs) or codec""s) of telephone line interface circuits are PSTN line-powered circuits that also require isolation from low-voltage power supplies. Because of this required isolation, a line powered interface circuit will not function until it is connected to a power source, i.e., the interface circuit must be activated when it is needed to connect the line side to the device side. When a circuit on the line side is placed in the xe2x80x9con-hookxe2x80x9d state (e.g., a telephone receiver is placed in its cradle) the local loop is opened and almost all of the power to the interface circuit is cut off. Activating the DAA or codec requires that some power must be drawn from the loop or from another source. While in the on-hook state a small amount of current (idle-state loop current) can be drawn for a short period of time from the TIP/RING line to register that an event has occurred on the PSTN TIP and RING line. However, it is extremely inefficient to draw the extra power unless it is really needed, i.e., it is extremely wasteful of time and resources to power up the DAA or codec when a transient noise signal occurs on the TIP and RING line.
The primary events that the interface circuit must detect while in the on-hook state are the application of a TIP and RING signal and a polarity reversal of TIP and RING DC voltage, either of which may be used to signal the need for more power. A problem can arise because ordinary transients on the line can have electrical characteristics very similar to those of actual events. To discriminate between transients and actual events, prior art interface circuits employ discrimination circuitry that determines the exact nature of every signal introduced thereto, but these additional circuits draw higher amounts of power. While drawing additional power is acceptable on a temporary basis, the amount of power available is limited. Discrimination circuits must operate at power levels no higher than the amounts determined by regulatory agencies as xe2x80x9csatisfactoryxe2x80x9d on-hook leakage currents.
In addition to determining what an event is, the event signals themselves must be transmitted from the device side to the line side. A problem arises because high voltage isolation prevents the transmission of the event signals from the device side to the line side. Prior art circuits avoid this problem by using optical couplers to transmit the signal caused by the actual event to the low voltage interface. These circuits employ a general purpose optical coupler with an LED input and a photo-transistor output. These optical couplers require xe2x80x9clight pipesxe2x80x9d which are cavities on the chip between the emitter and the detector of the optical coupler to allow the light to pass between the two. These light pipes increase the size and cost of the interface circuit. Capacitive coupling, another known isolation method, allows the circuit size to remain small and low cost, but the rate at which the events occur are too slow to accurately transmit them from the device side to the line side using capacitive coupling.
To avoid powering up the discrimination circuitry except when it is needed to process an actual event, it would be desirable to employ a line interface circuit which can distinguish between actual events (e.g., a ring signal, a polarity reversal, an audio signal) and noise (transient spikes on the line caused by a variety of sources, e.g., lightning, battery noise, etc.) before invoking the high power circuitry such as discrimination circuitry.
Accordingly, there remains a need for a simplified, smaller, and less costly telephone line interface circuit that can operate on the minimal amount of power available when the circuit is in the on-hook state to preliminarily distinguish between actual and noise events, and transmit event signals between the device side and line side while maintaining electrical isolation between the device side and the line side.
A telephone line interface circuit with event detection capabilities is provided that screens out transient signals and provides an indication to the line side that an actual event (actual data signal) has occurred so that appropriate discrimination circuitry is powered up to determine the exact nature of the actual event only when needed.
The telephone line interface circuit comprises a low voltage circuit including an event receiver, a line powered codec including an event transmitter, and an isolation circuit coupled between the low voltage circuit and the line powered codec. The low voltage circuit is coupled to the device side (host) of the network and the line powered codec (via sensory circuit) is coupled to the telephone line source of the network.
The line interface circuit switchabley operates in three modesxe2x80x94a low power mode, a medium power mode, and a full-power mode. The line interface circuit operates in the low power mode (sleep mode) unless the sensory circuit detects an input signal on the telephone line source. This input signal includes actual data signals and noise. Upon the receipt of an input signal, the line interface circuit switches to the medium power mode. The event transmitter and event receiver located in the line interface circuit develops an AC signal that represents the events (actual data signals) to be detected. This AC signal is timed to determine its sustained rate. The sustained rate has a timing threshold which is higher than a transient that occurrs on the line. Thus, the telephone line interface circuit checks the input (incoming on the telephone line source) signal to check whether it meets the threshold timing requirements. If the input signal does not require threshold timing, it is disregarded as being transient and no action is taken. This avoids the need to invoke the discrimination circuits that have large power requirements until they are actually needed. If the incoming signal meets the threshold requirement, the line interface circuit switches to the full power mode and facilitates the actual data transmission.